Currently the Warning header field is a Response header field:
14.46 Warning
The Warning response-header field is used to carry
additional information about the status of a response
which may not be reflected by the response status code.
This information is typically, though not exclusively,
used to warn about a possible lack of semantic transparency
from caching operations.
However, in section 13.1.2, it is said that
A proxy MUST NOT modify or add any of the following fields
in a response that contains the no-transform Cache-Control
directive, or in any request:
o Content-Encoding
o Content-Range
o Content-Type
A non-transparent proxy MAY modify or add these fields in
a response that does not include no-transform, but if it
does so, it MUST add a Warning 114 (Transformation applied)
if one does not already appear in the response.
The Warning header field must be a general header field to be able to
indicate that a message in a PUT has been modified. This can for example be
used to support advanced image processing in the proxy which is not
available in the client doing the PUT. I admit that the above wording is
quite confusing. I would say:
A proxy MUST NOT modify or add any of the following fields
in a message that contains the no-transform Cache-Control
directive:
o Content-Encoding
o Content-Range
o Content-Type
A non-transparent proxy MAY modify or add these fields to
a message that does not include no-transform, but if it
does so, if not already present, it MUST add a Warning 114
(Transformation applied).
This would also make it a lot easier to use the Warining header field by
Mandatory, for example.
Henrik
--
Henrik Frystyk Nielsen,
World Wide Web Consortium
http://www.w3.org/People/Frystyk